The problem of video copying cinema movies is known in the art. Several methods to prevent illegal copying have been suggested in the art. US-2003/063361, for instance, describes an optical state modulation method. The method comprises the steps of periodically modulating luminance of a visible light in temporal domain so as to generate an optical state variation on a recorded image that is obtained by image-capturing of a displayed image, the visible light being superposed on an original display image to produce the displayed image, the optical state variation being independent of an original display image and generating no hampering effect when the displayed image is directly watched.